Scanning electron microscope

ABSTRACT

An object of the present invention is to provide a scanning electron microscope suitable for monitoring apparatus conditions of the microscope itself, irrespective of the presence of charge-up, specimen inclination, and the like. In order to achieve the object, proposed is a scanning electron microscope including a function to monitor the apparatus conditions on the basis of information obtained with an electron beam reflected before reaching a specimen. Specifically, for example, while applying a negative voltage to the specimen to reflect the electron beam before the electron beam reaches the specimen, and simultaneously supplying a predetermined signal to a deflector for alignment, the scanning electron microscope monitors changes of the detected positions of the reflected electrons of the electron beam. If the above-mentioned predetermined signal is under the condition where an alignment is properly performed, the changes of the detected positions of the electrons reflect deviation of an axis.

BACKGROUND OF THE INVENTION

The present invention relates to an electron microscope that measures,inspects or observes a specimen by use of an electron beam, andparticularly, relates to a scanning electron microscope provided with afunction to make an axis adjustment of an electron beam, or a scanningelectron microscope suitable for measurement of a height or charge-up ofa specimen.

Size reduction and high integration of semiconductor devices have beenrapidly advanced, and length measurement and inspection techniques haveincreasingly become more important these days. Scanning electronmicroscopes are an apparatus that observes a surface of a specimen byscanning the specimen with a focused electron beam and detectingsecondary electrons or reflected electrons. Providing high resolution,the scanning electron microscopes are widely used as semiconductorlength measurement and inspection apparatuses which are represented by aCD-SEM (Critical Dimension-Scanning Electron Microscope), a DR-SEM(Defect Review-Scanning Electron Microscope), or the like.

In order to observe the specimen with high resolution using such anapparatus, the conditions for the apparatus need to be adjusted asappropriate. For example, when a trajectory of an electron beam isdeviated from the center of an objective lens, aberration is generatedand thus image quality deteriorates. To avoid this, optical axisadjustment needs to be made before observation. Resolutions ofrespective apparatuses vary due to a difference among the apparatuses,i.e., a so-called machine difference, which poses a problem in improvingmeasurement repeatability between the apparatuses. The conventionaltechniques for diagnosing and adjusting the apparatus conditions includefollowing methods.

Patent document 1 discloses a method in which: a particular point suchas an end of a knife edge or a center point of a cross mark is firstlymeasured at multiple focus levels; and the position of an objective lensaperture is automatically adjusted so that the specific points measuredat the respective focus levels overlap each other.

Patent document 2 proposes a charged particle beam device which makes afocus evaluation or a focus adjustment before changing the deflectingconditions of an alignment deflector; or which is provided with a tableof an amount of the focus adjustment corresponding to the deflectingconditions of the alignment deflector, and makes the focus adjustment inaccordance with the table after changing the deflection conditions ofthe alignment deflector.

The above-mentioned semiconductor inspection and measurement apparatusesare designed to be installed in a semiconductor production line and tooperate without human intervention. Moreover, for a speedup ininspection and length measurement, it is necessary to reduce eachprocessing time and to capture a clear image at a high speed withoutfocus deviation.

Generally, a focus adjustment is made by using a method of performing anautomatic focus adjustment on the basis of SEM images captured atdifferent focus planes. However, the method requires time, and thusthroughput decreases. In order to reduce the time required for a focusadjustment, Patent document 3 discloses a method of focusing accordingto a wafer surface height by detecting the surface height with anoptical height detector. Furthermore, patent document 4 discloses amethod of making an adjustment to correct focus deviation caused bycharge-up. In this method, the charge-up voltage is measured with anelectrostatic potentiometer before actual measurement, and a focusadjustment is made on the basis of the measured voltage and the heightof the specimen measured with an optical height detector.

In addition, patent document 5 discloses a technique in which: scores ofimages obtained by using different beam energies are analyzed; and thefocus adjustment is made by adjusting the beam energy in accordance withthis analysis.

Patent document 1: Japanese Patent Application Publication No.2005-276639

Patent document 2: Japanese Patent Application Publication No.2007-141632 (corresponding to US2007/0120065)

Patent document 3: Japanese Patent Application Publication No. 11-149895(corresponding to U.S. Pat. No. 6,107,637)

Patent document 4: Japanese Patent Application Publication No.2005-338096 (corresponding to U.S. Pat. No. 6,946,656)

Patent document 5: Japanese Patent Application Publication No.2001-236915 (corresponding to U.S. Pat. No. 6,521,891)

SUMMARY OF THE INVENTION

Patent document 1 and patent document 2 describe the examples in whichan axis adjustment (hereinafter, may be referred to as an alignment) ismade using images captured by a scanning electron microscope. However,if the specimen is charged up due to electron beam scanning at the timeof acquiring the image, the electron beam is bent. For this reason, analignment may not be performed properly. Moreover, some of semiconductorspecimens such as wafers have their surfaces inclined. Image changesgenerated by such inclination, charge-up or the like of the specimenmake it difficult to determine which one of factors such as theinclination and charge-up causes deviation of the axis. As a result, aproper alignment is difficult in some cases.

This description proposes later on a scanning electron microscope aimingat monitoring apparatus conditions of the microscope itself,irrespective of the presence of charge-up, inclination or the like of aspecimen, and proposes, as one example of a specific aspect of suchmicroscope, a scanning electron microscope capable of performing aproper alignment on the basis of the monitored result.

The technique disclosed in patent document 3 has a problem when atransparent specimen is observed. More specifically, in such a case, thelens may not be focused on the specimen because the height of thespecimen measured with the optical height detector is different from theactual height of the specimen. The technique also has another problemthat even when the height of the specimen is accurately measured, theelectron beam is accelerated (or decelerated) by charge-up, so that thefocus deviates. In the technique disclosed, as a method of adjusting thefocus to correct focus deviation caused by the charge-up, in patentdocument 4, the charge-up voltage is measured with an electrostaticpotentiometer before actual measurement. Measurement of the voltagebefore actual measurement is a very effective method from a viewpoint ofthroughput. However, in the case where the charge-up voltage changesover time or in a similar case, a focus adjustment requires little bitmore time than that in the case where the voltage does not change atall.

In the technique disclosed in patent document 5, the scores of theimages obtained using the different beam energies are analyzed, and afocus adjustment is made by adjusting the beam energy in accordance withthis analysis. However, since the images are obtained through theelectron beam irradiation of the specimen, a damage or chargeaccumulation of the specimen may be caused due to the electron beamirradiation, and the throughput may be reduced due to delay in the focusadjustment.

This description proposes below a scanning electron microscope aiming ataccurately measuring the potential or the height of a specimen whilesuppressing a damage or the like of the specimen caused by irradiationwith the electron beam.

In order to achieve the above-mentioned aims, hereinafter, proposed is ascanning electron microscope provided with a function to monitorapparatus conditions of the microscope itself, on the basis ofinformation obtained in a state where an electron beam is prevented fromreaching the specimen. More specifically, for example, while applying anegative voltage to the specimen to reflect the electron beam before theelectron beam reaches the specimen and simultaneously supplying apredetermined signal to a deflector for alignment, the microscopemonitors changes of the detected positions of the above-mentionedreflected electrons obtained. If the predetermined signal is under thecondition where an alignment is properly performed, changes of thedetected positions of the above-mentioned electrons reflect deviation ofthe axis. Since the electron beam is prevented from reaching thespecimen at this time, it is possible to monitor deviation of the axiswhile suppressing the charge-up caused by the electron beam irradiation.

With the above-mentioned configuration, it is possible to monitor theapparatus conditions in the state where the electron beam is preventedfrom reaching the specimen, and to highly precisely set the apparatusconditions.

Furthermore, in order to attain the other aim mentioned above, proposedis a scanning electron microscope that measures the height and thepotential of a specimen by using a detection result of charged particlessuch as the electrons obtained by irradiating the specimen with acharged particle beam such as an electron beam, or the like whileapplying a voltage to the specimen in order to prevent the chargedparticle beam from reaching the specimen. Moreover, in one aspectthereof proposed herein, the scanning electron microscope corrects theapparatus conditions (for example, magnification, focus, observationcoordinate, and the like) on the basis of the measured height andpotential of the specimen, since the apparatus conditions change due tothe charge-up of the specimen.

With the above-mentioned configuration, the height or the potential ofthe specimen can be measured while the electron beam is prevented fromreaching the specimen. Accordingly, it is possible to adjust theapparatus conditions with high precision without allowing the apparatusconditions to change due to a damage or charge accumulation of aspecimen that might be otherwise caused by the electron beamirradiation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a scanning electron microscope forperforming a potential measurement method by use of mirror electrons;

FIG. 2 is a diagram illustrating a trajectory of the mirror electrons;

FIG. 3 is a diagram illustrating a method of calculating the potentialof the specimen based on the trajectory of the mirror electrons;

FIG. 4 is a diagram illustrating a method of calculating, based on thetrajectory of the mirror electrons, deviation of an axis of an electronbeam from an objective lens;

FIG. 5 is a schematic flow to calculate, based on the trajectory of themirror electrons, deviation of the axis of the electron beam from theobjective lens;

FIG. 6 is a schematic flow to calculate, based on luminance of themirror electrons, deviation of a position of a field emission electrodefrom an aperture;

FIG. 7 is a schematic flow to calculate, based on the trajectory of themirror electrons, deviation of a magnification;

FIG. 8 is a diagram illustrating a method of calculating, based on animage of the mirror electrons, deviation of a position of a structurewithin the apparatus;

FIG. 9 is a schematic configuration diagram of a scanning electronmicroscope;

FIG. 10 is a diagram illustrating a relationship among a characteristicquantity extracted from a detection result of the mirror electrons, thepotential of the specimen, and the height of the specimen;

FIG. 11 is a diagram showing a relationship between the potential andthe height of the specimen when the characteristic quantity extractedfrom the detection result of the mirror electrons takes a specificvalue.

FIG. 12 is a diagram illustrating a method of deriving the potential andthe height of the specimen from the multiple characteristic quantities;

FIG. 13 is a diagram illustrating a seventh embodiment and a tenthembodiment;

FIG. 14 is a diagram illustrating an eighth embodiment;

FIG. 15 is a diagram illustrating a ninth embodiment;

FIG. 16 is a diagram illustrating a specimen information computingdevice;

FIG. 17 is a diagram illustrating an optical condition control device;and

FIG. 18 is a diagram illustrating a twelfth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

Hereinafter, description will be given of a method for accuratelydiagnosing apparatus conditions in distinction from a state of aspecimen by using a reflected electron beam that is not in contact withthe specimen, and given of an apparatus for implementing the method.

First, a measurement principle will be described. In a scanning electronmicroscope including: a lens including an electrode and a magneticfield, and focusing an electron beam by accelerating and deceleratingthe electron beam; an alignment deflector that performs axialcorrection; an objective lens; an aperture; and a stage that holds aspecimen and applies a potential to the specimen, acceleration energy ofelectrons is defined as Ee, the potential applied to the specimen isdefined as Vr, and |−Vr| is set so as to be larger than Ee.

When the electron beam is emitted to the specimen in this state, theincident electrons are reflected right above the specimen withoutentering the specimen, as shown in FIG. 2 (this state is referred to asa mirror state, the reflected electrons are referred to as a mirrorelectrons, and a virtual reflection plane is referred to as a mirrorsurface). The reflected mirror electrons travel within the lens systemin a reverse direction. When a detector is disposed in the lens system,the detector can detects arrival positions (Xm, Ym, Zm) of the mirrorelectrons. Here, for example, when a Vr setting value is changed tochange mirror surface position, deviation of the mirror electron arrivalposition corresponding to the amount of the position change can bedetected.

A charge-up voltage of the specimen can be determined from this amountof deviation. For example, by maintaining multiple data groups of Xmwhen changing Vr, a correlation function in which an abscissa shows Vrand an ordinate shows Xm can be created as shown in FIG. 3. If thespecimen is charged up at this time, the position of the mirror surfacevaries corresponding to the charge-up amount. Accordingly, even when Vris set to Vr1, the arrival position of the mirror electrons deviatesfrom Xm1 to Xm2 or the like. A value of Vr estimated from Xm2 and thecorrelation function is Vr2. Since Vr2 reflects the position of themirror surface, Vr2 shows the potential of the charged specimen surface.Namely, the charge-up amount Δφ can be calculated from Vr2−Vr1. Here,Ym, Zm and the like can also be used in place of Xm, for the correlationfunction.

In the description below, the above-mentioned method is applied and amethod for diagnosing an apparatus will be provided, in which each ofsetting parameters (alignment deflector setting value, condensing lenssetting value, deflecting coil setting value, and the like) is definedas a variable in place of Vr, and change in conditions (positiondeviation of apparatus configuration components, electrode and magneticfield lens, axial correction amount, magnification, and the like) of theapparatus is calculated using a correlation function acquired underreference conditions. For example, deviation of the axis of the beamfrom the objective lens can be calculated in the following manner.

The setting value of the alignment deflector is defined as Ia. In thereference conditions in which an axis adjustment is made (in a statewhere a current Ia1 is supplied to the alignment deflector) as shown inFIG. 4, the arrival position (Xm, Ym, Zm) of the mirror electron isacquired under each of different optical conditions of an opticalelement (for example, voltage Vr applied to the specimen, currentI_(obj) supplied to the objective lens (voltage Vobj when the objectivelens is an electrostatic lens), and the like). Then, a correlation curvebetween the arrival positions and the optical conditions is calculated.

Next, under the apparatus conditions in which the axis of the beam isdeviated, the above-mentioned Ia is varied in order to calculate theabove-mentioned correlation function. The correlation function iscalculated for each of the different las. In this state, when a currentvalue of the alignment deflector is set to a predetermined value of Ia1at the time of diagnosis, the detected position of the mirror electronin a state of having no axis deviation is Xm1 because the positionagrees with the correlation function under the reference conditions.

When the axis is deviated, the detected position of the mirror electronis, for example, Xm2, and this position agrees with a differentcorrelation function. Here, the deviation amount of the axis at the timeof diagnosis can be estimated using the amount of axis deviationcorresponding to the agreed correlation function. Alternatively, whenthere is no agreed correlation function, the deviation amount of theaxis can be estimated, for example, with the interpolation between twocorrelation functions which are adjacent to the detected position.

If a table showing a relationship between the deviation amount of thebeam axis and an Ia appropriate value is prepared in advance, Ia can beeasily corrected using the estimated deviation amount of the beam axis.As a variable of the correlation function, Vb, Io, or the like can bealso used in place of Ia. The present method is also applicable todetermination of a deviation direction of the beam axis as well as thedeviation amount of the beam axis.

Hereinafter, using the drawings, more detailed description will befurther given of a method of monitoring the apparatus conditions by useof the mirror electrons.

FIG. 1 shows a method for diagnosing an apparatus by use of the mirrorelectrons, and an example of an electron microscope on which theapparatus is mounted. A primary electron beam 1 is extracted by applyingan extraction voltage between a field emission electrode 11 and anextraction electrode 12. The extracted primary electron beam 1 isaccelerated by an acceleration voltage that an acceleration voltagecontrol system 44 applies to an accelerating electrode 13. The primaryelectron beam 1 is subjected to scanning deflection by a condensing lens14, an upper deflector 21, and a lower deflector 22. Deflectionintensity of the upper deflector 21 and that of the lower deflector 22are adjusted so that two-dimensional scanning on a specimen 23 may beperformed with a lens center of an objective lens 17 serving as afulcrum. The deflected primary electron beam 1 is accelerated by anacceleration cylinder 18 provided in a path of the objective lens 17.The accelerated primary electron beam 1 is focused by lens action of theobjective lens 17. A stage voltage control system 48 applies a potentialVr to the specimen (or specimen stage (also, referred to as a specimenstand) 24), the potential Vr being sufficiently larger than accelerationenergy Ee of the electrons.

For example, when Ee is 2 keV, a potential Vr larger than approximately−2000 V is applied. Thereby, an equipotential surface (mirror surface 2)of −2000 V in potential is obtained in a position above the specimen.The primary electron beam is reflected here, and returns upwards. Thiselectron will be referred to as a mirror electron 3. The mirror electron3, which has passed through the lens system, reaches a detector 29. Thedetector 29 detects a position (Xm, Ym, Zm) of the mirror electron 3. Astorage unit 45 is a storage unit that records this information on (Xm,Ym, Zm). A computing element 40 calculates an amount of ‘deviation’ fromthe information on (Xm, Ym, Zm) on several conditions recorded in thestorage unit 45, and thus calculates an amount of changes in apparatusconditions on the basis of this amount of ‘deviation.’ This informationis sent to an analyzer 41, so that the analyzer 41 controls theapparatus conditions therein and sets signals of parameters of a controlsystem. The control system includes, for example, an alignment deflectorcontrol system 42, an objective lens control system 43, a deflectorcontrol system 46, and the like, which control the apparatus conditions.A guide 20 is disposed and installed so as to surround a trajectory ofthe primary electron beam 1.

FIG. 5 is a schematic flow that shows an example of a method forcalculating deviation of the beam from the objective lens center at thetime of diagnosis based on deviation of the mirror electron trajectory.A setting value of an alignment deflector 25 is defined as Ia, and thedetected position of the mirror electron is defined as (Xm, Ym, Zm). In501, the apparatus is adjusted to the reference conditions, and is setto a mirror state. The setting value of the alignment deflector 25 atthis time is defined as Ia1. In 502, Ia is changed, data on Xm (data onYm or Zm may be used) is acquired, and the correlation curve between Iaand Xm is recorded in the storage unit 45. Here, more accuratecorrelation curve can be obtained as more data groups of Ia and Xm arerecorded. Subsequently, in 503, setting of the alignment deflector 25 ischanged in a way that the deviation amount of the beam axis isintentionally changed, and a correlation curve between Ia and Xm isacquired. Here, the multiple changed deviation amounts of the beam axisare acquired. The correlation function of the reference conditions andmultiple correlation functions each having a different deviation amountof the beam axis are stored in the storage unit 45 in 504. At the timeof diagnosis of the apparatus condition, in 505, the apparatusparameters are set to the same values as those of the referenceconditions so that the mirror state is made, and the detected positionof the mirror electrons is calculated in 506. In 507, the detectedposition of the mirror electrons is compared with the correlationfunction on the reference conditions stored in the storage unit 45.Then, it is determined that when the detected position is on thecorrelation curve, the beam is not deviated at the time of diagnosis,and when the detected position is not on the correlation curve, the beamis deviated. Alternatively, deviation of the beam from the referenceconditions is determined by acquiring the correlation curve at the timeof diagnosis and comparing that with the correlation curve on thereference conditions. In 508, in comparison with the multiplecorrelation functions stored, the deviation amount of the beam axis withwhich the correlation function at the time of diagnosis agrees iscalculated in comparison with the stored correlation functions. If thereis no data on an agreed correlation function, interpolation between anytwo correlation functions next to each other is made to estimate thedeviation amount of the beam axis. In 509, reference is made to thetable of appropriate setting values for the alignment deflector relativeto the deviation amount of the beam axis prepared in advance, acorrection amount of the setting value for the alignment deflector iscalculated from calculated inclination of the beam, and the correctedsetting value is outputted to the alignment deflector control system 42.This corrects deviation of the beam axis from the objective lens center.

Embodiment 2

Hereinafter, a second embodiment will be shown. Based on deviation ofthe mirror electron trajectory, position deviation from the aperture ofthe field emission electrode at the time of diagnosis is calculated. Inthe same manner as the above-mentioned embodiment, the potential appliedto the specimen is set so as to be sufficiently larger than theacceleration energy Ee of the electrons, and a position Xt of the fieldemission electrode 11 can be adjusted. In this state, the number ofdetected mirror electrons where the setting value of the condensing lensis Ic1 is defined as Bm1. Since the mirror electron is not in contactwith the specimen, Bm1 is equal to the number of electrons that havepassed through the aperture. Where the setting value of the condensinglens is Ic2, the beam is emitted with the same energy, and the numberBm2 of the detected mirror electrons at this time is calculated from thedetector. When multiple data groups of (Ic, Bin) are acquired, thecorrelation curve between Ic and Bm can be calculated from this data.The correlation curve between Ic and Bin is acquired on the referenceconditions in which the position of the field emission electrode havingits Xt adjusted does not deviate. If position deviation is generated inthe field emission electrode, the number of the electrons passingthrough the aperture changes. Consequently, the correlation curvebetween Ic and Bm changes. Therefore, position deviation of theelectrolytic emission electrode can be determined from this change incorrelation curve. At this time, instead of Bm, a mirror electrondetected position (Xin, Ym, Zin), and the like can be also used forobtaining the correlation curve.

Embodiment 3

FIG. 6 is a schematic flowchart showing a third embodiment, which is anexample of a method for diagnosing position deviation of an electronsource. A setting value of the condensing lens is defined as Ic, and thenumber of the mirror electrons detected per unit time is defined as Bm.In the same manner as in the above-mentioned schematic flow, in 601, theapparatus is adjusted to the reference conditions, and is set to themirror state. In 602, when Ic is changed, the number of the electronsthat passes through the aperture changes. Accordingly, the correlationfunction between Ic and Bm can be acquired from data of Bm. In 603, thecorrelation curve between Ia and Bm is stored in the storage unit 45.Next, in 604, the mirror state is set at the time of diagnosis of theapparatus conditions. In 605, the correlation function between Ia and Bmis acquired in the same manner as in the case of the referenceconditions. In 606, the correlation function with the referenceconditions is compared with the correlation function at the time ofdiagnosis. If both the correlation functions agrees with each other, itis determined that there is no position deviation between the fieldemission electrode and the electrode, and when both do not agree, it isdetermined that position deviation has occurred. With the adjustment ofXt so that both the correlation functions may agree with each other in607, it is possible to reproduce the state where there is no positiondeviation.

Embodiment 4

FIG. 7 is a flowchart showing a fourth embodiment, which is an exampleof a method for diagnosing change in magnification by use of the mirrorelectrons. In the same way as the above-mentioned embodiment, in 701,the potential applied to the specimen is set to a value sufficientlylarger than the acceleration energy Ee of the electrons, a deflectionintensity ratio of the upper deflector 21 and the lower deflector 22 isadjusted to the reference conditions, and the detected position of themirror electrons when a setting value of a deflection amount is Id isdefined as (Xm, Ym, Zm). In 702, the correlation function of Id and Xm(or Ym, Zm) on the reference conditions is acquired. Multiplecorrelation functions each having Id changed are acquired in 703, andare stored in the storage unit 45 in 704. In 705, the mirror state isset at the time of diagnosis of the apparatus conditions, and thedetected position of the mirror electron is acquired in 706. In 707,determination is made that there is no change in magnification when theacquired Xm (or Ym, Zm) agrees on the correlation function, and thatthere is a change in magnification when the acquired Xm (or Yin, Zm)does not agree on the correlation function. In 708, the deflectionamount that agrees on the correlation function is calculated, and in709, the calculated deflection amount is fed back to the deflectorcontrol system 46. Thereby, change in magnification is corrected.

Embodiment 5

A method for calculating change in intensity of an objective lens at thetime of diagnosis, which is a fifth embodiment, can be brought intopractice by replacing the variable Id of the correlation function with asetting value Io of the objective lens in FIG. 7.

Embodiment 6

Hereinafter, a sixth embodiment will be described. Similarly to theabove-mentioned embodiment, the potential applied to the specimen is setto be sufficiently larger than the acceleration energy Ee of theelectrons. Then, the primary electron beam 1 extracted by applying theextraction voltage 13 between the field emission electrode 11 and theextraction electrode 12 is accelerated in the condensing lens 14, andpasses through the aperture 15. Then, the primary electron beam 1 isreflected above the specimen to become the mirror electron 3, andreturns to the detector 29.

Here, when the mirror electron is scanned by the upper deflector 21 andthe lower deflector 22, structures within the apparatus are irradiatedwith the mirror electron. Therefore, for example, when the detectors aredisposed as a detector L and a detector U as shown in FIG. 8A, an imagereflected from shapes of the structures can be acquired. In this case,from the image acquired by the detector 1, the shape of the structure A,the shape of the structure B, and a positional relation therebetween canbe calculated, as shown in FIG. 8B. From the image acquired by thedetector U, the shape of the structure B, the shape of the structure C,and the positional relation therebetween can be calculated, as shown inFIG. 8C. Use of these images enables diagnosis of the machine differencebetween apparatuses, for example. Although the method can beaccomplished also by using secondary electrons discharged from thespecimen surface, the mirror electron has an energy width smaller thanthat of the secondary electron and can be formed into a fine flux.Therefore, use of the mirror electron enables clearer determination ofthe structure.

Use of the mirror electron allows the apparatus diagnosis within acolumn, with the mirror electron not being in contact with the specimen.Accordingly, it is possible to reduce correction of apparatus conditionchanges over time and the machine difference between the apparatuses bycalculating an appropriate adjustment value from the diagnosis result.

Embodiment 7

Hereinafter, description will be given of an apparatus that measures thepotential and the height of the specimen using the detection result ofthe mirror electron and that automatically corrects the apparatusconditions (magnification, focus, observation coordinate) that changedue to specimen charge-up. The mirror electron is obtained in a statewhere the voltage is applied to the specimen being irradiated with acharged particle beam, in such a way that the charged particle beamfails to reach the specimen (hereinafter, this state may be referred toas the mirror state). Although the description below will be given, asan example, of a scanning electron microscope, which is an example of acharged particle beam apparatus, the present embodiment is alsoapplicable, for example, to a focused ion beam apparatus which is otheraspect of the charged particle beam apparatus. In this case, when theion beam has positive charge, a positive voltage is applied to thespecimen in order to form the above-mentioned mirror state. In addition,the detected charged particles are also cations.

First, a method for measuring the potential and the height of thespecimen will be described. Under mirror conditions in which the primaryelectron beam does not enter the specimen, optical parameters(parameters related to lens magnification such as an object point ZC tothe objective lens, an exciting current I_(obj) of the objective lens,and the potential of the specimen Vs=Vr+ΔVs, and a boosting potentialVb) are respectively set to an arbitrary value. Then, to calculate thepotential and the height of the specimen, direct or indirect measurementis performed on a beam arrival position A_(H) (X_(H), Y_(H), Z_(D))(arriving point of an H trajectory on the detector) depending on a beamdivergence angle on an object surface, a beam arrival position A_(G)(X_(G), Y_(G), Z_(D)) (arriving point of a G trajectory on the detector)depending on a beam position on the object surface, or both of A_(H) andA_(G). A method for deriving the arrival position of the mirror electronwill be shown below.

Any electron detector can be selected as a detector for mirror electron,from among a detector that directly detects the mirror electron, such asMCP, a detector that causes the mirror electron to collide with areflector or the like and detects the discharged secondary electrons,and other electron detectors. However, it is desirable to use a detectorhaving multiple detecting elements two-dimensionally spread. Moreover,the arrival position of the mirror electron may be determined usingelements obtained by adhering a fluorescent screen to a light sensingportion of a CCD (Charge Coupled Device).

It is possible to calculate the arrival position or distribution of themirror electrons from output signals of these several detectingelements, and to calculate deviation from a reference value as acharacteristic quantity. Moreover, use of images enables easierdetection of the characteristic quantity. When a mirror electron isreflected right above the specimen and passes through the inside of thelens system, the mirror electron is influenced by a path of the beam, orthe structure. In order to acquire the image, a position of an incidentbeam may be scanned. Thereby, the shape of the structure in the beampath is formed as an image. The characteristic quantity Fm can bemeasured by measuring a dimension of the shape of the structuretransferred into the image, and sags of an edge.

Relationships (F1 (Vs, Zs), F2 (Vs, Zs), . . . ) among thecharacteristic quantity, the potential of the specimen, and the heightof the specimen under multiple optical conditions (optical condition 1,optical condition 2, . . . ) are stored in advance as functions or asvalues calculated by simulation or actual measurement, and are referredto at the time of measurement. Then, the potential and the height of thespecimen can be derived from the characteristic quantities (Fm1, Fm2 . .. ) acquired under the multiple optical conditions.

Here, an example of a method for deriving the height and the potentialof the specimen will be described, using a case where optical parameters(optical condition 1, optical condition 2) other than the potential Vsand the height Zs of the specimen are set, and the characteristicquantities (Fm1, Fm2) are acquired.

The relationship among the acquired characteristic quantity Fm, thepotential Vs of the specimen, and the height Zs of the specimen is shownin FIG. 10. The characteristic quantity Fm varies depending on thepotential Vs and the height Zs of the specimen. First, description willbe given of a case where the optical condition 1 is set and the acquiredcharacteristic quantity is Fm1. When the characteristic quantity is Fm1(dotted line in the drawing), the derived potential varies depending onthe height of the specimen. For example, when the height of the specimenis ZA (ZB, ZC), the measured potential is VA (VB, VC). A relationshipbetween the height and the potential of the specimen at this time (onthe dotted line) is shown in FIG. 11. In this way, the height Zs and thepotential Vs of the specimen depend on each other. Therefore, when oneof Zs and Vs is found, the other can be also found. Furthermore, whenanother optical condition (optical condition 2) is set and thecharacteristic quantity is thus acquired, both of the height Zs and thepotential Vs of the specimen can be measured.

A method for measuring the height Zs and the potential of the specimenwill be described, using, as an example, a case where the characteristicquantity acquired under the optical condition 1 (optical condition 2) isF1 (F2). FIG. 12 shows an explanatory view of a method of measurement. Asolid line (dotted line) shows the relationship between the height Zsand the potential Vs of the specimen when the characteristic quantityacquired under the optical condition 1 (optical condition 2) is F1 (F2).Two curves intersect at this time. When calculating this point ofintersection, the potential and the height of the specimen can bederived.

Here, the method for calculating an individual curve that shows therelationship between the potential Vs and the height Zs of the specimenfor each optical condition, and calculating the potential and the heightof the specimen from the calculated curve is shown. However, F1 (Vs, Zs)and F2 (Vs, Zs) may be represented by a certain function and the heightZs and the potential Vs of the specimen may be thus calculated bydirectly solving an equation. Alternatively, the characteristic quantityFm may be created into a table using the height Zs and the potential Vsof the specimen, and the height Zs and the potential Vs of the specimenmay be calculated by interpolation or fitting by use of points thatexist adjacent to the acquired characteristic quantity Fm.

When the functions F1 (Vs, Zs) and F2 (Vs, Zs) have a higher ordercomponent, the functions have multiple solutions. However, by limiting arange in which these solutions exist, or increasing the number of thecharacteristic quantities acquired under different optical conditions,measurement of the potential and the height of the specimen can beperformed with stability.

Moreover, a setting width of the optical conditions in the mirror modecan be expanded by installing structures for determining thecharacteristic quantity above and under the deflector. Although a methodfor calculating the height Zs and the potential of the specimen using adimension of a shadow (projection image) of a first structure and adimension of a shadow of a second structure will be shown here, themethod can be also applied to other characteristic quantity.

When an upper detector 922 detects the mirror electrons, an on-screenlength L1 of the shadow of the first structure is determined by themagnification M_(obj) of the objective lens, and can be represented bythe following formula:

$\begin{matrix}{{{\overset{->}{I}}_{1} = {M_{obj}\begin{pmatrix}{Xo} \\{Yo}\end{pmatrix}}}{{{1/L}\; 1} = {\overset{->}{I}}}} & (1)\end{matrix}$

where

X_(O), Y_(O): deflection amount on the object surface, X_(O)=C1X_(DEF),Y_(O)=C1 Y_(DEF)

X_(DEF), Y_(DEF): deflection amount of the deflector and the inverse ofthe length L1 of the shadow of the first structure is proportional tothe magnification M_(obj) that varies depending on the potential and theheight of the specimen, and other optical parameters.

Next, a length L2 of a shadow of the second structure varies dependingon the magnification of the objective lens and the deflection amount ofthe deflector, as shown in the following formula:

$\begin{matrix}{{{\overset{->}{I}}_{2} = {{M_{obj}\begin{pmatrix}X_{O} \\Y_{O}\end{pmatrix}} + \begin{pmatrix}X_{DEF} \\Y_{DEF}\end{pmatrix}}}{{{1/L}\; 2} = {{\overset{->}{I}}_{2}}}} & (2)\end{matrix}$

Accordingly, when a ratio (L1/L2) of the length L1 of the shadow of thefirst structure to the length L2 of the shadow of the second structureis extracted as the characteristic quantity, a magnitude of thecharacteristic quantity can be varied to a desired deflection amount,independent of the deflection amount of the deflector. For this reason,the setting width of the optical conditions in the mirror mode can beexpanded.

Moreover, the potential and the height of the specimen are derived fromthe characteristic quantity obtained from the mirror electrons, andappropriate values suitable for observation conditions are inputted intothe optical parameters, such as a holder potential Vr, an excitingcurrent I_(obj) of the objective lens, energy E of a primary beam, andan observing magnification. Thereby, a clear SEM image with highmagnification accuracy can be obtained, without irradiating the specimenwith the beam before observation.

In the scanning electron microscope according to the present embodiment,two types of optical modes can be set: an optical mode for observationand an optical condition (mirror mode) of the mirror mode in which thepotential of the specimen is set in a way that the primary electron beamcannot reach the specimen. Before performing normal SEM imageobservation, the optical condition is set to the mirror mode. Moreover,on the basis of the detection result of the mirror electrons obtained inthe mirror mode, the potential and the height of the specimen aremeasured, and the optical parameters in the optical mode for SEM imageobservation, such as the amount I_(obj) of excitation of an objectivelens 912 and the potential Vr of the specimen holder, are set from themeasured result. Consequently, without irradiating, with the electronbeam before observation, the specimen whose charge-up and height areunknown before observation of the SEM image, a clear SEM image havinghigher magnification accuracy can be acquired also for the specimen.Hereinafter, detailed description will be given using the drawings.

FIG. 9 shows an apparatus configuration of the SEM. The potential of aspecimen holder 905 is set to a value which prevents the primaryelectron beam from reaching a specimen 904, and the optical condition isset to the mirror mode. In a case where arrival energy of the electronbeam to the specimen is 2000 eV, for example, in a state where novoltage is applied to the specimen 904, or the specimen holder 905 (maybe referred to as a specimen stand) (that is, a state where the specimenhas an earth potential (except the case where charge-up is adhered)),when the voltage applied to the specimen is larger than 2 kV, theelectron beam fails to reach the specimen, and is reflected by apotential barrier right in front of the specimen. Such a state isreferred to as a mirror state, and to provide the mirror state by anadjustment of the optical conditions of the electron microscope (anacceleration voltage, the voltage applied to the specimen, and the like)is referred to as a mirror mode.

Behavior of the primary electrons in the mirror mode will be shownbelow. A primary electron beam 902 is extracted from a field emissionelectrode 901. The extracted primary electron beam 902 is accelerated byan accelerating electrode not shown. For example, a voltage Vacc isapplied to the accelerating electrode.

The primary electron beam 902 is focused by a condensing lens 911 andsubjected to scanning deflection by an upper deflector 906 and a lowerdeflector 907. The deflected primary electron beam 902 is furtheraccelerated by a boosting electrode 908 provided in a path of theobjective lens 912. The accelerated primary electron beam is deceleratedunder the influence of an electric field formed due to a potentialdifference between an electric field control electrode 923 and thespecimen 904, and the resultant primary electron beam is reflected rightabove the specimen. The reflected primary electron beam 903 travelsbackward within a mirror body. By setting the voltage Vr applied to thespecimen to be larger than the above-mentioned Vacc, the electron beamis reflected in a direction reverse to an electron beam irradiationdirection without reaching the specimen.

The primary electron beam that has traveled backward is accelerated bythe boosting electrode 908, and passes through a first structure 913 andrushes into the lower detector 921. When the lower detector 921 isturned OFF, the primary electron beam continues travelling backward, issubjected to deflection action by the deflector, passes through a secondstructure, and is detected by the upper detector 922. The detectedsignal is inputted into a specimen information computing device 9120,and the potential Vs and the height Zs of the specimen are derived.Then, the calculated potential Vs and height Zs of the specimen areinputted into an optical condition control device 9110. In the opticalcondition control device 9110, a voltage and exciting currentappropriate for an exiting coil and an electrode, such as the electricfield control electrode, the boosting electrode, an electrode whichdetermines optical properties of the objective lens, the condensinglens, the upper deflector, the lower deflector and the like, are set ina way that focusing is made above the observed specimen based on theinputted potential and height of the specimen and the observationconditions. Thereby, an SEM image is acquired.

Here, an example is shown when the mirror electron is detected by theupper detector and the height and the potential of the specimen arecalculated in the specimen information computing device 9120. However,detection of the mirror electrons by the lower detector 921 isadvantageous in that the mirror electron is not influenced by the upperdeflector 906 and the lower deflector 907 at the time of a return trip.Without deviating from the spirit of the invention, the presentembodiment can be applied to cases where the first detector detects thecharacteristic quantity, where only the second detector detects thecharacteristic quantity, where the characteristic quantity detected bythe first and second detectors are used, and the like.

Next, a seventh embodiment will be described using a flowchart in FIG.13. In the present embodiment, description will be given of an electronmicroscope having a reference data recording unit 9122 that records afunction Fm (Zs, Vs) showing a relationship among the height of thespecimen, and the potential of the specimen, and the characteristicquantity extracted from the detection result of the mirror electrons, orthat records a table of the characteristic quantity (see FIG. 16).

In Step 100, the optical condition is set to the mirror mode. In Step110, the mirror electron is detected, and the detection result isinputted into the specimen information computing device 9120. In Step120, the characteristic quantity is extracted by a characteristicquantity extracting part 9121 from the inputted detection result of themirror electrons. In Step 130, a specimen potential and heightcalculation part 9123 derives the height and the potential of thespecimen from the extracted characteristic quantity and the table of thecharacteristic quantity or the function Fm (Zs, Vs) showing therelationship among the characteristic quantity, the height of thespecimen, and the potential of the specimen, the function or the tablebeing recorded in advance for each optical condition of the mirror modein the reference data recording unit 9122.

In Step 140, the derived height Zs and potential Vs of the specimen, anda desired observation condition are inputted into an optical conditioncalculation part 9111 illustrated in FIG. 17. Based on the inputtedparameters, calculation is made on the optical parameters (potential ofeach electrode, amount of excitation of each exiting coil, deflectionamount of the deflector, observation magnification, and the like) whichcause the electron beam to focus above the specimen. The calculatedoptical parameters are inputted into the optical condition setting part9112, and the potential and exciting current for each electrode and theexciting coil are thus set respectively.

The above-mentioned characteristic quantity denotes the detectedposition of the electron on the detector, the dimension of the structureon the image, the position of the structure, sags of the edge, theamount of rotation, luminance, and the like. All of them can be detectedusing existing techniques. For example, the dimension of the structureis detected based on the magnification of the electron microscope, anoccupation percentage of the structure on the image, and the like; theposition of the structure based on general image processing techniquesfor identifying the position of the structure on the image; sagging ofthe edge based on a sharp evaluation that is used for a focusadjustment, and the like; the amount of rotation, about degrees ofrotation of the target structure on the image, based on general imageprocessing techniques; and the luminance based on luminance histogramformation of the image.

Embodiment 8

A method for deriving the height and the potential of the specimen usingthe characteristic quantity extracted from the shadow of the structurewill be shown as an eighth embodiment. The present embodiment shows acase where the structure in a shape of 1 is installed in the position ofthe first structure 913, and the mirror electron is detected by theupper detector 922 while the electron beam is scanned by the upperdeflector 906 and the lower deflector 907. Alternatively, the positionalrelation of the detectors or the structures may be changed. At thistime, as shown in FIG. 14, the image having characteristics depending onthe shape of the structure is obtained from the detection result of themirror electron. The dimension of the structure, sags of the edge, theamount of rotation, and the luminance thus obtained are converted intonumbers as the characteristic quantity, and the height and the potentialof the specimen are derived.

Embodiment 9

As a ninth embodiment, an electron microscope will be shown, in which astructure installed on mirror electron trajectory is installed bothabove and below a deflector, and based on a ratio or difference of thecharacteristic quantities extracted from the shadows of the structuresinstalled above and below the deflector, the potential and the height ofthe specimen are measured. A first structure 913 in a shape of 1 isinstalled below the deflector, and the second structure 914 in a shapeof “2” is installed above the deflector. While the electron beam isscanned by the upper deflector 906 and the lower deflector 907, themirror electron is detected by the upper detector 922. Thereby, theimage as shown in FIG. 15 can be acquired. When a ratio of the dimensionof the first structure and the dimension of the second structure, whichare displayed on the image, is extracted as the characteristic quantityat this time, there is an advantage in that a magnitude of thecharacteristic quantity is not dependent on the exciting current of adeflecting coil. While the ratio of the dimension of the first structureand the dimension of the second structure is used as the characteristicquantity here, values relating to sagging of the edge, the amount ofrotation, and the luminance may be extracted as the characteristicquantity.

Embodiment 10

As a tenth embodiment, an electron microscope that measures the heightand the potential of the specimen from the characteristic quantitiesacquired under the optical condition in multiple mirror modes will beshown. If the number of the characteristic quantities has not reachedyet the desired number at the time of checking the number of thecharacteristic quantities at Step 160 in the flowchart shown in FIG. 13,the electron microscope sets the optical condition to a new mirror mode,and extracts a new characteristic quantity in Step 170.

Additionally, in Step 120, two or more characteristic quantities, forexample, sags of the edge and the dimension of the structure, a beamdiameter on a detecting surface and a deflecting width, or the like, maybe extracted from the detection result of the mirror electron under asingle optical condition.

Embodiment 11

As an eleventh embodiment, an electron microscope that holds acalibration specimen on the specimen holder will be shown. Thiscalibration specimen is used to ensure an absolute value of thepotential or focus conditions. It is desirable that the calibrationspecimen should be a conductor, located approximately at the same heightas the observed specimen, and grounded to the specimen holder so as tohave the same potential as that of the specimen holder. When a referencedata is acquired in advance from the calibration specimen, an error ofmeasurement generated by changes of the optical conditions over time canbe reduced.

Moreover, use of materials, such as Au whose changes over time are smalland whose work function is known, is advantageous in that an absolutevalue of a specimen surface can be ensured.

Embodiment 12

In a twelfth embodiment, an electron microscope that holds, on a waferholder, a calibration specimen for adjusting changes of the opticalcondition over time and updates a measured value or function stored inthe reference data recording unit 9122 will be shown.

FIG. 18 shows a flowchart of the twelfth embodiment. Determination ismade as to whether reference data is updated in Step 200. The referencedata may be periodically updated, or may be automatically updated whenfocus deviation at the time of observation becomes remarkable. In Step210, the measurement target is changed to a calibration specimen forreference data. In Step 220, the mirror mode in which the reference datais updated is set. In Step 230 and Step 240, the mirror electron isdetected and the characteristic quantity is extracted. In Step 250, theheight of the calibration specimen is measured from a Z sensor, anexciting current of normal SEM observation, and multiple characteristicquantities extracted in the mirror mode.

In Step 260, the relationship among the height of the specimen, thepotential of the specimen, and the characteristic quantity obtained fromStep 210 to Step 250 is stored in the reference data recording unit9122.

Moreover, using a calibration specimen having multiple heights, thereference data is acquired under each of the optical conditions ofmultiple heights and in multiple mirror modes. Thereby, even whenchanges over time are generated in the optical condition in the mirrormode, the height and the potential of the specimen can be accuratelymeasured. Additionally, before SEM image observation, the opticalconditions such as focus, magnification, and observation position, canbe adjusted without irradiating the specimen with the electron beam.

1. A scanning electron microscope, comprising: a detector that detectselectrons; an optical element that adjusts optical conditions of anelectron beam; a deflector that makes an axis adjustment of the electronbeam; a control device that controls a negative voltage applied to anyone of a specimen and a specimen stand; and a detector that detects aposition of a trajectory of electrons discharged from the specimen,wherein the control device controls the negative voltage to create astate where the electron beam is reflected before reaching the specimen,stores in advance a position of a trajectory of the electrons obtainedby the detector in the state when a predetermined signal is supplied tothe deflector, and detects a change between the stored position and aposition obtained by supplying the predetermined signal to the detector.2. The scanning electron microscope according to claim 1, wherein thecontrol device stores, in advance, a correlation between the opticalconditions obtained and the position obtained by the detector whenconditions of the optical element are changed.
 3. The scanning electronmicroscope according to claim 2, wherein the control device stores aplurality of correlations respectively for a plurality of signalssupplied to the deflector.
 4. The scanning electron microscope accordingto claim 1, wherein the control device varies the signal supplied to thedeflector according to the changes.
 5. A scanning electron microscope,comprising: a lens that accelerates and decelerates an electron beam andfocuses the electron beam; an alignment deflector that makes an axisadjustment of the lens; and a stage that holds a specimen and applies apotential to the specimen, wherein the scanning electron microscopeincludes a detector that causes incident electrons to be reflected rightabove the specimen by setting a potential applied to the specimen to belarger than that of the accelerated electron beam, and detects atrajectory of the reflected electrons, and the scanning electronmicroscope detects a change in the trajectory obtained by the detector.6. The scanning electron microscope according to claim 5, wherein asignal value supplied to the alignment deflector is corrected on thebasis of a change between a trajectory of the electrons detected when apredetermined signal is supplied to the alignment deflector, and atrajectory of the electrons detected and stored in advance when thepredetermined signal is supplied to the alignment deflector.
 7. Ascanning electron microscope, comprising: a detector that detectselectrons; a specimen stand that supports a specimen that is irradiatedwith an electron beam; and a control device that controls a negativevoltage applied to the specimen or a specimen stand, wherein the controldevice controls the negative voltage so that the electron beam isreflected before reaching the specimen, and calculates the height of thespecimen on the basis of a characteristic quantity of the electronsdetected by the detector and a relationship between the characteristicquantity and the height of the specimen.
 8. The scanning electronmicroscope according to claim 7, wherein, a structure with which a partof the reflected electrons collides is disposed between the detector andthe specimen, and the structural member is projected on the detector. 9.The scanning electron microscope according to claim 8 wherein, aplurality of the structures are disposed above and below a scanningdeflector that scans the electron beam.
 10. The scanning electronmicroscope according to claim 9, wherein the control device calculatesthe height of the specimen on the basis of a ratio betweencharacteristic quantities of projected images of the structures disposedabove and below the scanning deflector.
 11. A scanning electronmicroscope comprising a stage which applies a potential to a specimen,wherein the scanning electron microscope is settable in a mirror mode inwhich incident electrons are reflected right above the specimen, byapplying a potential which prevents an electron beam from reaching thespecimen, the potential and the height of the specimen are derived onthe basis of a plurality of characteristic quantities extracted from adetection result of mirror electrons reflected right above the specimen,and at least any one of an excitation amount and a display magnificationof an objective lens, and a scanning region is adjusted based on thederived height and potential of the specimen.
 12. The electronmicroscope according to claim 11, wherein a structure is installed on atrajectory of the mirror electrons, and the height and the potential ofthe specimen are measured based on characteristic quantities extractedfrom a shadow of the structure reflected in the detection result of themirror electrons.
 13. The electron microscope according to claim 12,wherein a plurality of the structures installed on the trajectory of themirror electrons are installed above and below a deflector, and thepotential and the height of the specimen are measured on the basis of aratio between the characteristic quantities extracted from the shadowsof the structures installed above and below the deflector, the shadowsbeing reflected in the detection result of the mirror electrons.
 14. Theelectron microscope according to claim 11, wherein the height and thepotential of the specimen are measured from the characteristicquantities acquired under optical conditions in a plurality of mirrormodes.
 15. The electron microscope according to claim 11, wherein two ormore characteristic quantities are extracted from the detection resultof the mirror electrons in one of the mirror modes, and the height andthe potential of the specimen are measured based on the extractedcharacteristic quantities.
 16. The electron microscope according toclaim 11, wherein a potential-height calibration specimen for ensuringan absolute value of the potential is held on a specimen holder or inthe electron microscope.
 17. The electron microscope according to claim16, comprising, a recording device that records optical conditions inone or more of mirror modes, and records a function and a table whichshow a relationship among the characteristic quantity, the height of thespecimen, and the potential of the specimen, the characteristic quantityextracted from the detection result of the mirror electrons obtained ina set mirror mode, wherein any one or both of the potential and theheight of the specimen is derived using an acquired characteristicquantity, and any one of the function and table recorded in therecording device.
 18. The electron microscope according to claim 16,wherein a measurement is automatically and manually moved to thecalibration specimen, and the measurement result is calibrated byacquiring the function and the table which show the relationship amongthe characteristic quantity, the height of the specimen, and thepotential of the specimen obtained from the detection result of themirror electrons from the calibration specimen.
 19. The electronmicroscope according to claim 16, wherein the calibration specimen has aplurality of heights.
 20. The electron microscope according to claim 16,comprising, a mechanism that changes the height of the calibrationspecimen.